1. Field of the Invention
This invention relates generally to ceramic materials, and more particularly to ceramic materials for insulators of spark plugs.
2. Description of the Prior Art
Spark plugs typically extend into the combustion chamber of an internal combustion engine and include a cylindrical metal shell having a firing end and a hook shaped ground electrode attached thereto. A cylindrical insulator is disposed partially within the metal shell and extends axially beyond the metal shell toward the firing end and also toward a terminal end. At the firing end, a cylindrical center electrode is disposed within the insulator and projects axially out of the insulator toward the ground electrode, wherein a spark gap is defined between the electrodes.
During operation, ignition voltage pulses are applied though the spark plug to the center electrode, thereby causing a spark to jump the spark gap between the center electrode and ground electrode. The spark ignites an air and fuel mixture within the combustion chamber to create high temperature combustion to power the engine. The ceramic insulator is used to isolate the high voltage at the electrodes, ensuring that the spark happens at the tip of the center electrode and not anywhere else on the spark plug. The insulator is subjected to harsh conditions at temperatures of up to about 1000° C., and therefore, is typically formed of a ceramic material to provide adequate thermal conductivity and dielectric strength.
However, as combustion engines become more complex, operating at high temperatures, and requiring higher voltages for the spark to jump the spark gap, the performance demands for ceramic insulators increase. In addition, manufacturers desire to increase the service lifetimes of spark plug insulators while keeping the size and cost of the spark plug insulator to a minimum.